Initial conditions of the universe : signatures in the cosmic microwave background and baryon acoustic oscillations.

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Abstract

In this thesis, we investigate the signatures of isocurvature initial conditions in the cosmic microwave
background (CMB) through the temperature and polarization anisotropies, and in the
large-scale structure distribution through the baryon acoustic oscillations (BAO).
The first part of this thesis is a brief review of the standard cosmological model with its underlying
linear cosmological perturbation theory. We supplement it with a general discussion on the
initial conditions of the primordial fluctuations.
In the third chapter, we review the evolution of the perturbations in the adiabatic model. We
focus on the evolution of adiabatic perturbations in the photons and baryons from the epoch of
initial conditions to the photon-baryon decoupling, as these determine the main features of the
primary CMB anisotropies and of the baryon acoustic oscillations.
The fourth chapter recalls the theory of the CMB anisotropies in the adiabatic model. We consider
the perturbations from the last scattering surface and evolve them through the line of sight
integral to get the adiabatic CMB power spectrum. We review the effect of different cosmological
parameters on the adiabatic CMB temperature spectrum.
In the fifth chapter, we investigate the observational signatures of the isocurvature perturbations
in the CMB anisotropies. We first derive simple semi-analytic expressions for the evolution of
the photon and baryon perturbations prior to decoupling for the four isocurvature regular modes
and show that these modes excite different harmonics which couple differently to Silk damping
and alter the form and evolution of acoustic waves. We study the impact of different cosmological
parameters on the CMB angular power spectrum through the line of sight integral and find
that the impact of the physical baryon and matter densities in isocurvature models differ the most
from their effect in adiabatic models.
In the last two chapters, we explore in detail the effect of allowing for small amplitude admixtures
of general isocurvature perturbations in addition to the dominant adiabatic mode, and
their effect on the baryon acoustic oscillations. The sixth chapter focuses on the distortion of
the standard ruler distance and the degradation of dark energy constants due to the inclusion of
isocurvature perturbations, while the seventh chapter discusses in more detail the sensitivity of
BAO dark energy constraints to general isocurvature perturbations. We stress the role played by
Silk damping on the BAO peak features in breaking the degeneracy in the peak location for the
different isocurvature modes and show how more general initial conditions impact our interpretation
of cosmological data in dark energy studies. We find that the inclusion of these additional
isocurvature modes leads to a significant increase in the Dark Energy Task Force figure of merit
when considered in conjunction with CMB data. We also show that the incorrect assumption of
adiabaticity has the potential to substantially bias our estimates of the dark energy parameters.
We find that the use of the large scale structure data in conjunction with CMB data significantly
improves our ability to measure the contributions of different modes to the initial conditions.